Filter units for combining signals in radio base stations are conventionally built up of various units. FIG. 1 shows an example of a combiner unit 10 that is arranged within a chassis 13 consisting of a resonator 15 and a tuner 14, which is movably arranged within said resonator 15. The tuner 14 is adjusted to a position relative to a resonator axis, in the figure denoted the z-axis, in order to achieve a certain resonator frequency. This adjustment is often performed by means of a motor unit 11 and a threaded shaft 12 that is connected to said motor unit 11 and inserted into a threaded hollowness of the tuner 14 or other wise connected to it such that the radial movement of the shaft 12, which is caused by the motor unit 11, can be transformed into a linear movement of the tuner 14 along said resonator axis. This arrangement, however, achieves a non-linear frequency tuning and provides insufficient precision for frequency adjustments.
A tuning arrangement according to the state of the art may consists of a resonator 15 of a first dielectric material comprising a hollowness within which a tuner 14 of cylindrical shape and consisting of a second dielectric material can be inserted. The tuner 14 is movable arranged along an axis 12 of displacement, in this example z-axis, and can be moved within a range from a first position that corresponds to a maximum insertion into the hollowness of the resonator 15 to a second position where the tuner has been completely protruded out of said resonator. For the sake of simplicity, tuner movements are only considered in direction of the positive z-axis. However, it is apparent that is would be likewise possible to adjust the resonator frequency for tuner movements in the opposite direction.
FIG. 2 illustrates a sketch of the distribution of the electrical field for a TE01δ-mode in a resonator 31 comprising a hollowness 32 within which a tuner could be inserted. It can be observed that the field strength in the resonator hollowness is relatively weak; hence the perturbation of the field in the hollowness allows a tuning of the resonator frequency in a selected band. The resonator frequency depends on the dielectric properties of the building block consisting of resonator and tuner, in particular on the choice of the dielectric materials and the amount of the tuner mass that is interposed in the resonator hollowness. Frequency adjustments are achieved by varying the amount of dielectric material within the resonator hollowness. The main influence results from the resonator while the variation of the tuner position is applied for precision adjustments of the desired resonator frequency. For instance, each tuner position within the resonator implies a certain amount of dielectric material in the resonator hollowness and corresponds thus to a certain resonator frequency. The size of the frequency change depends on the amount and the dielectric properties of the protruded part of the tuner. The resonator frequency increases as long as the tuner is protruded out of the resonator hollowness within the tuning area.
A known system for tuning high-frequency dielectric resonators has been presented in EP 0 492 304. Said system comprises a male dielectric resonator having an external diameter d that penetrates to a certain degree p into a female dielectric resonator having an external diameter D. U.S. Pat. No. 4,728,913 shows another dielectric resonator which is capable of adjusting the dielectric resonator frequency through a wider frequency range without deteriorating Q0.